Snap co-founders Evan Spiegel (R) and Bobby Murphy walk to ring the opening bell of the New York Stock Exchange, March 2, 2017.

Shares of social media firm Snap were downgraded for a second time this week Friday, adding to an already dismal trading week for the stock.

Analysts at Cowen lowered their rating to market perform from outperform and cut their price target to $17 a share from $21. John Blackledge, an analyst at Cowen, said the downgrade came as a result of a lower ad sales forecast:

We cut our '17-'22 ad revenue forecast given the competitive environment within Digital advertising and new monetization efforts like Direct Response and Self Serve, which are ramping in 2H17 and into 2018, likely taking longer to play out.

The stock has plunged nearly 8.7 percent this week to break below its initial public offering price of $17 a share. Snap shares closed Thursday's session at $15.69 and were lower by 1 percent in premarket trading Friday.

Cowen's Blackledge also raised concerns about Snap's user growth, noting it is not a "slam dunk, particularly with other players with more scale, like FB and particularly Instagram Stories, competing for users and their finite time."

"Lastly on the monetization front, it remains extremely early days as SNAP really only started to monetize its user base in '15/'16 mostly in the US, while many of SNAP's European/ROW offices opened in late '16 and early '17," Blackledge said.